1. Field of the Invention
The present invention relates to a fluid flow meter and in particular to a flow meter of the type that operates by measuring changes in electrical properties of an obstruction in the fluid flow path in order to determine the fluid flow.
2. Description of the Prior Art
A known flow meter of this type has an obstruction, in the form of a rigid paddle, to which is attached a piezo-resistive rod. The paddle is arranged to be deflected by a force exerted by a flowing fluid so that it tends to twist the piezo-resistive rod. This twisting results in changes in the electrical resistance of the rod, Changes in the electrical properties (such as resistance) of electrical circuitry that includes the rod are then measured and are used to calculate the fluid flow.
One disadvantage of the known device is that, particularly when used to measure small flows, the paddle must be made relatively large in order to construct a sufficiently sensitive flow meter. This adds an appreciable and undesirable resistance to the flowing fluid. Additionally, as the mass of the paddle increases its resonant frequency decreases. This may lead to a reduction in the usefulness of the sensor in measuring oscillatory flows, since oscillatory flows with frequencies close to and above the resonant frequency cannot be easily or accurately measured using such a paddle.
Moreover, the paddle produces anxe2x80x9caveragexe2x80x9d flow measurement from across the entire surface that faces upstream of the flow. This can lead to inaccurate measurements, especially when part of this surface lies in a region of turbulent flow.
An object of the present invention is to provide a fluid flow meter wherein at least some of the problems associated with known flow meters are alleviated.
The above object is achieved in accordance with the principles of the present invention in a fluid flow meter having a fluid flow path therein with an obstruction disposed in the fluid flow path which is deflectable in response to fluid flow, the obstruction having electrical characteristics which vary dependent on the degree of deflection, and wherein the obstruction is formed by a number of flexible tines which are oriented so as to deflect in the direction of fluid flow. A monitor is operably coupled to the obstruction so as to monitor changes in electrical characteristics caused by deflection of the tines, and to calculate the fluid flow therefrom.
By employing number of tines instead of a single, large paddle, the mass of each deflectable element is considerably reduced which enables a meter to be constructed having a faster response time and which is better suited to the measurement of oscillatory flows. Moreover, since the tines deflect in the direction of the fluid flow the resistance the sensor offers to flowing fluid decreases as the flow increases, reducing turbulence effects which may occur at higher flows.
The degree of flexion of each tine in the flowing fluid can be separately selected and can be used to provide anxe2x80x9caveragexe2x80x9d flow value using all of the tines. Alternatively, a selective flow value can be obtained using one or more of the tines chosen, for example, because the variations in determined flexes indicates that they lie outside regions of turbulent flow, or because the tines are of differing degrees of flexibility and those most responsive at the extant flow rate are selected.
Preferably, the tines are integral with a semi-conducting substrate, such a silicon, on to which other sensors and circuitry is integrated to form a flow meter that can be accurately and reproducibly fabricated using standard etching and doping techniques well established in the field of integrated chip technology.
Most usefully, the tines may be disposed to form a variable area obstruction across an aperture in the substrate and a differential pressure sensor can be integrated with the substrate to calculate the flow rate from pressure differences measured upstream and downstream of the tines in a manner common in the art, for example in the ways described in U. S. Pat. No. 4,006,634, the disclosure of which is incorporated herein by reference. With this arrangement two flow meters, both dependent on the flexibility of the tines, are incorporated in the same device, with practically no additional space. In such an arrangement one meter can be used as a safety backup device for the other.